FIG. 9 shows a structure of a conventional even-harmonic mixer disclosed in, for example, JP Patent No. 2795972. This even-harmonic mixer is an example of the even-harmonic mixer for a down converter that receives a high-frequency signal (having a frequency frf) and a local oscillation signal (having a frequency flo) and outputs a local frequency of a difference between frf and an even-number multiple of flo as an intermediate frequency signal (frequency fif).
In FIG. 9, reference numeral 1 designates a high-frequency signal terminal, reference numeral 2, a local oscillation signal terminal, reference numeral 3, an intermediate frequency signal terminal and reference numerals 4a and 4b, diodes. These diodes 4a and 4b are connected in parallel, with polarities inverted, thereby constituting an anti-parallel diode pair 5. Further, reference numeral 6 designates a lowpass filter to pass the intermediate frequency signal and reference numeral 7, a bandpass filter that passes the high-frequency signal, the filters both being connected to a first terminal 8 of the anti-parallel diode pair 5. Further connected to this first terminal 8 is an open stub 10 of an approximately quarter wavelength (¼λ) at the local oscillation signal frequency. On the other hand, a short stub 11 of approximately ¼λ at the local oscillation signal frequency is connected to a second terminal 9 of the anti-parallel diode pair 5. It is to be noted that the lowpass filter 6 and the bandpass filter 7 constitute a branching filter 12.
The even-harmonic mixer for a down converter having the aforementioned construction operates as follows. That is, a local oscillation signal of the frequency flo is inputted through the local oscillation signal terminal 2 and applied to the second terminal 9 of the anti-parallel diode pair 5. On the other hand, a high-frequency signal of the frequency frf is inputted through the high-frequency signal terminal 1 and applied to the first terminal 8 of the anti-parallel diode pair 5 via the bandpass filter 7. Then, a mixture wave having a frequency fout given by the next equation is generated across the anti-parallel diode pair 5:fout=|frf±2mflo|                where m: an integer,and a signal of the frequency fif given, for example, by the following equation:fif=frf−2mflois selected from the mixture waves and outputted as an intermediate frequency signal from the intermediate frequency signal terminal 3.        
In this case, if the frequency fif of the intermediate frequency signal is sufficiently lower than the frequency frf, then the connection point 8 connected to the open stub 10 of approximately ¼λ for the local oscillation signal frequency flo functions as an open to the high-frequency signal frf and as a short circuit to the local oscillation signal flo. On the other hand, the connection point 9 connected to the short stub 11 of approximately ¼λ with respect to the local oscillation signal frequency flo functions as a short circuit to the high-frequency signal frf and as an open to the local oscillation signal flo. The local oscillation signal and the high-frequency signal can be isolated from each other. Moreover, the even harmonic signal of the local oscillation signal is canceled in the anti-parallel diode pair 5 and therefore outputted from nowhere.
It is also possible to make the even-harmonic mixer for a down converter having the aforementioned construction function as a mixer for an upconverter that receives an intermediate frequency signal at the intermediate frequency signal terminal 3 and a local oscillation signal at the local oscillation signal terminal 2 and outputs a high-frequency signal from the high-frequency signal terminal 1.
However, the aforementioned conventional even-harmonic mixer has the following problems. That is, it is required to additionally connect a bandpass filter for allowing the high-frequency signal inputted to the high-frequency signal terminal 1 to pass, and this increases the costs of the components and of the mounting process. Moreover, when the milliwave band frequency is very high, a loss at a connection between the even-harmonic mixer and the bandpass filter becomes significant, and the characteristic is deteriorated.
On the other hand, for the purpose of reducing the costs for the components and the mounting process and reducing the loss at the connection point between the even-harmonic mixer and the bandpass filter, it is conceivable to form the even-harmonic mixer and the bandpass filter on one semiconductor substrate to thereby integrate them into one semiconductor chip. However, in such a case, if the higher harmonic mixer and the bandpass filter are separately designed and integrated by simply being arranged on a semiconductor chip and connected together by means of a transmission line of an appropriate length, the passive circuit of the open stub 10 and the short stub 11 of the even-harmonic mixer, the filters 6 and 7 and so on disadvantageously will occupy the greater part of the area. Accordingly, there is a problem that the area of the semiconductor chip is increased and the unit price of the semiconductor chip is raised.